I do it with an almost 3 year old.
First time - "we do not poke babies. if you poke the baby again, you will have to sit on the step by yourself."
Second time - "i told you that if you poke the baby again, you will have to sit on the step. Go and sit there" (if she doesn't go, take her gently by the hand, lead her to the step and sit her down). "sit there until I come to get you"
She will sit there for 2 minutes. If she gets up, I sit her back down, tell her again why she has to sit there and start the clock again. Had to repeat this a lot in the first couple of days.
Two minutes is up - "why did you have to sit on the step? Are you ready to say sorry?" she will say "don't poke baby" and then cuddles and says sorry to me and the baby.
Don't call it the naughty step - ours is called 'time-out'. Does sound a bit American, but I think 'naughty' just sounds so negative.
If your girl is under 2.5 she may be having trouble understanding exactly what she has done wrong, so this might not work until she's closer to 3.